A.) Field of Use
The present invention relates to catalysts, systems, and methods that are useful for treating an exhaust gas which occurs from combusting hydrocarbon fuel—more particularly exhaust gas containing nitrogen oxides, such as an exhaust gas produced by diesel engines, gas turbines, or coal-fired power plants.
B.) Description of Related Art
Exhaust gas is emitted when fuels such as natural gas, gasoline, diesel fuel, fuel oil or coal is combusted and is discharged into the atmosphere through an exhaust pipe, flue gas stack or the like. The largest portions of most combustion exhaust gas contain relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2); but the exhaust gas also contains in relatively small part noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). Of particular relevance to the present invention is an exhaust gas containing NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), that is derived from lean burn engines such as diesel engines for mobile applications.
Often, systems for treating diesel engine exhaust gas include one or more catalyst compositions coated on or diffused into a substrate to convert certain or all of the noxious and/or toxic exhaust components into innocuous compounds. One such conversion method, commonly referred to as Selective Catalytic Reduction (SCR), involves the conversion of NOx in the presence of a catalyst and with the aid of a reducing agent into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant, typically anhydrous ammonia, aqueous ammonia, or urea, is added to an exhaust gas stream prior to contacting the catalyst. The reductant is absorbed onto a catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for a stoichiometric reaction using either anhydrous or aqueous ammonia for an SCR process is:4NO+4NH3+3O2→4N2+6H2O2NO2+4NH3+3O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Known SCR catalysts include zeolites or other molecular sieves disposed on a monolithic substrate. Molecular sieves are microporous crystalline solids with well-defined structures and generally contain silicon, aluminum and oxygen in their framework and can also contain cations within their pores. A defining feature of molecular sieves is that their frameworks are made up of interconnected networks of molecular tetrahedrals. Aluminosilicate molecular sieves, for example, are arranged as an open network of corner-sharing [AlO4]- and [SiO4]-tetrahedrals. In the case of a silica tetrahedral, a silicon atom is at the center of the tetrahedral while the four surrounding oxygen atoms reside at the tetrahedral's corners. Two or more tetrahedrals can then be linked together at their corners to form various crystalline structures.
A molecular sieve framework is defined in terms of the geometric arrangement of its primary tetrahedral atoms “T-atoms” (e.g., Al and Si). Each T-atom in the framework is connected to neighboring T-atoms through oxygen bridges and these or similar connections are repeated to form a crystalline structure. Since the framework per se is merely the arrangement of these coordinated atoms, specific framework types do not expressly depend on composition, distribution of the T-atoms, cell dimensions or symmetry. Instead, a particular framework is dictated solely by the geometric arrangement of T-atoms. (Codes for specific framework types are assigned to established structures that satisfy the rules of the IZA Structure Commission.) However, materials of differing compositions, but arranged according to the same framework, can possess very different physical and/or chemical properties.
Crystalline structures can be formed by linking individual unit cells of the same or different frameworks together in a regular and/or repeating manner. These crystalline structures may contain linked cages, cavities or channels, which are of a size to allow small molecules to enter—e.g. the limiting pore sizes can be between 3 and 20 Å in diameter. The size and shape of these microporous structures are important to the catalytic activity of the material because they exert a steric influence on the reactants, controlling the access of reactants and products.
Of particular interest to the present invention are small pore molecular sieves, such as those having a chabazite (CHA) framework. Two particular materials that have CHA frameworks, the aluminosilicate SSZ-13 and the silicoaluminophosphate SAPO-34, are known to be useful in SCR processes for converting NOx to N2 and O2 and for other catalytic processes and each has separate advantages.
In addition to their porosity, molecular sieves often have other elements introduced as extra-framework constituents to improve their catalytic performance. For example, U.S. Pat. No. 5,472,594 suggests that incorporating phosphorus into a ZSM-5 zeolite provides a composition having unique properties as a catalytic agent. However, the phosphorus described in the '594 patent is not present as a crystalline framework constituent i.e., it has not been substituted for silicon or aluminum atoms. Likewise U.S. Pat. No. 7,662,737 describes ZSM-5 having free phosphate and/or phosphates bonded to extra-framework aluminum. Other examples of extra-framework constituents include metals, such as copper or iron.
Thus, a need remains for improved hydrothermally stable small pore molecular sieves having a high degree of catalytic activity.